1. Field of the Invention
This invention relates to a drive axle suspension; and, more particularly, to a heavy-duty drive axle suspension capable of maintaining a substantially constant pinion gear angle while flexing to permit operationally sufficient independent movement of wheels coupled to the suspension.
2. Description of the Related Art
Heavy-duty truck drive axle suspensions typically comprise a pair of trailing arm suspension assemblies, each mounted parallel to and spaced from frame rails in the truck chassis. Each trailing arm suspension assembly comprises a trailing arm having one end pivotally mounted to a hanger bracket, which is rigidly mounted to one of the frame rails, and an air spring connecting the other end of the trailing arm to the frame rail. The suspension assemblies carry a drive axle differential comprising a housing from which extends a pinion gear and axle housings, containing the axle shafts. The axle housings connect the differential to the trailing arms through axle brackets. The pinion gear is connected to the engine through the drive shaft. The axle shafts mount the wheels and are driven by the engine through the connection between the drive shaft, pinion gear, and axle shaft.
A trailing arm suspension of this type translates road forces imparted to the wheels into a rotational movement of the trailing arms relative to the hanger brackets. The rotational movement of the trailing arm is cushioned by the air spring positioned between the end of the trailing arm and the frame rail.
A common design problem for drive axle suspensions is to keep the pinion gear parallel to the engine output shaft. The torque applied to the pinion gear from the engine through the drive shaft, results in torque applied to the drive tires which results in tractive effort being applied to the ground through the tire contact area. The reaction to the torque from the drive tires is a torque in the drive axle housing along its lateral axis, which is clockwise when viewed from the left side of the vehicle. This torque, when coupled to a single pivot suspension, tends to raise the forward end of the trailing arm and thus raises the frame a few inches with respect to the axle. This height rise changes the pinion angle dramatically.
The torque induced pinion angle change is exacerbated by newer high horsepower, high torque engines that produce substantially greater torque at lower rpms than previous engines. The new engines produce such high torque at such low rpms that each piston firing can result in a spike in the torque loading of the drive line components extending from the engine to the pinion gear of the differential. The magnitude of the torque load, in conjunction with a single pivot suspension, can alter the pinion angle dramatically, which sets up vibrations in the entire drive train. To prevent damage to drive line components and eliminate vibration, it is necessary to keep the pinion angle within predetermined limits.
One attempt to maintain the pinion angle at a substantially constant angle stiffened the suspension to prevent rotation around the suspension pivot in response to the torque reaction lifting force on the drive axle. One solution adds springs to the shock absorbers to prevent frame rise and subsequent pinion angle change.
The stiffening of the suspension to prevent the rotation of the axle housing can give rise to some additional undesirable operational characteristic. The suspension can be so stiff that it will reduce axle travel and, when the vehicle is lightly loaded and traversing slightly uneven ground, it may lose traction. Further, the spring in the shock absorber changes the ride characteristics and decreases the suspension's response over rough roads.
Therefore, it is desirable to have a drive axle suspension that maintains a substantially constant pinion angle while providing sufficient suspension flexibility to ensure the best possible performance and durability.